1. Field of the Invention
This invention relates to a magnetic recording medium. More particularly, the invention is concerned with a magnetic recording medium which satisfies electromagnetic conversion characteristics and typical physical requirements such as bonding strength, repeated-passage stability, still-playback characteristics, etc. without unfavorably affecting the smoothness on the surface.
2. Description of the Prior Art
The magnetic recording medium in common use today comprises a resin film or similar base material coated with a magnetic coating material comprising a mixture of a magnetic powder and a resin binder. Endeavors have been made in the art provide a magnetic recording medium much improved in characteristics. In particular, a variety of resin binders have been proposed for use in preparing magnetic coatings. The binders thus far most often employed have consisted of two-component combinations of urethane, vinyl cloride-vinyl acetate, cellulose, epoxy, ethylene, and ester resins. Those binary binders have, however, had a disadvantage in that, in the choice of the mixing ratio of the two components, electromagnetic conversion characteristics and physical properties such as repeated-passage stability and bonding strength required for the recording medium are antagonistic to each other. It is extremely difficult to select a binder with the correct component ratio to satisfy both requirements.
Nitrocellulose, proposed as a resin binder, has been widely employed because of the good wettability it confers on the magnetic powder and also for good leveling of the resulting coating. However, the high flammability inherent in nitrocellulose necessitates its careful handling at all times to provide protection against fire and explosion hazards. In order to secure safety in storage and in handling, the law obliges the supplier to impregnate nitrocellulose with isopropyl alcohol as a protective ingredient, in an amount from 30 to 35% of the weight of the ester, to protect against fire and explosion. On the other hand, when nitrocellulose is combined with a polyurethane compound or the like, non-nitrated hydroxyl groups in the cellulose chain will undergo cross-linking with isocyanate radicals in the urethane compound. In the application of nitrocellulose as a resin binder for magnetic coating material, the mechanical strength that stems from this cross-linking as well as the good affinity of nitrocellulose to magnetic coating material proves very helpful in improving electromagnetic conversion characteristics and physical properties required of the magnetic tape. However, before the reaction with the isocyanate radical of the urethane compound is taken advantage of, the hydroxyl group in the isopropyl alcohol with which the nitrocellulose is impregnated will react first with the isocyanate radical of the urethane compound added to effect the cross-linking with the nitrocellulose, thus reducing the degree of cross-linking and lessening the effect of improving the physical properties of the product. In an attempt to correct this drawback, it has been proposed to replace the isopropyl alcohol used for impregnation as a protective ingredient, by dibutyl terephthalate, dioctyl phthalate, or other similar plasticizer. The replacement is still disadvantageous because the plasticizer weakens the coating, rendering it difficult for the coating to have satisfactory physical properties. It is well-known in this connection that nitrocellulose offers as its physical features improved wear resistance and running stability but also has a disadvantage of low bonding strength.